Two novel semiconducting B₂ CO monolayers with high carrier mobilities

The design of new two-dimensional (2D) materials with moderate band gaps and high carrier mobility is an important aspiration for materials innovation. Recent studies have shown that boron and oxygen atoms can be integrated into the graphene lattice to form a stable B-C-O monolayer structure. To search for the most energetically stable configuration for 2D B-C-O, here, we theoretically propose two new 2D B-C-O crystal structures with a stoichiometric ratio of 2:1:1, namely monolayer (1 L) C_3v - and C_2v -B₂ CO. Two configurations have 0.09 and 0.03 eV/unit cell lower energies than the reported 1 L C_s -B₂ CO configuration (Nanoscale 2016, 8, 8910). This result is further confirmed by particle swarm optimization (PSO) calculations. According to the chemical bonding analysis, 1 L C_3v -B₂ CO with a quasi-planar configuration has the lowest energy, which is consisted of three strong B'-O σ-bonds, three B″-C σ-bonds, and one B'-C σ-bond. As a result, 2D B₂ CO has an ultra-high mechanical strength of ~366 J m^-2 , comparable to graphene ~352 J m^-2 . In addition, 1 L C_3v -B₂ CO is a semiconductor with an HSE06 bandgap of 2.57 eV, and it has a high electron mobility of up to ~150 cm²  v^-1  s^-1 . The high kinetic and thermodynamic stabilities of both 1 L C_3v - and C_2v -B₂ CO were confirmed according to phonon dispersion and molecular dynamic simulation. Comparable to that of crystalline silicon, 1 L C_3v -B₂ CO also shows a high light absorption intensity in the 400-550 nm region. Therefore, 2D C_3v -B₂ CO will have promising applications in semiconductor devices and photodetectors.